The Acquired Immunodeficiency Syndrome (AIDS) is an urgent world-wide clinical problem. Pneumocystis carinii pneumonia (PCP) is a major cause of morbidity and mortality in these patients. Inhaled pentamidine isethionate has been used both for treatment of and prophylaxis against Pneumocystis carinii pneumonia. However, the mechanism of action of pentamidine isethionate against Pneumocystis carinii remains unknown. Pulmonary alveolar macrophages are the primary resident host defense cell in the lung and are capable of releasing toxic oxygen radicals which can injure Pneumocystis carinii. We hypothesize that Pentamidine isethionate activates alveolar macrophages via signal transduction pathways initiated by increases in intracellular [Ca2+]i which activates the oxidase enzyme leading to the release of potent oxygen radicals such as superoxide anion (O2-) nitric oxide (NO) and peroxynitrite anion (ONOO) in vivo and in vitro. These activated macrophages also release IL-8 and TNF-a potent proinflammatory cytokines which have both direct and indirect actions on P. carinii. Inhaled pentamidine as prophylactic treatment against Pneumocystis carinii pneumonia is an attempt to target drug therapy in an organ specific, non-toxic manner. The development of a rat model that closely mimics the human situation allows us to evaluate the mechanism of action of pentamidine and define its effects on lung cells both in the short and long term. Determination of the mechanism of action of inhaled pentamidine may improve clinical management of immunosuppressed patients with Pneumocystis carinii pneumonia.